One of the questions I am always asked is about cannabis. People say cannabis is harmless, and they are right. For most people, cannabis is harmless, but if you happen to be one of those people with a family history of psychosis, cannabis is not harmless at all. It hugely increases your chance of developing schizophrenia.
People have been worried about cannabis for centuries. In fact, one of the oldest studies was called the Indian Hemp Corp, by the Indian Hemp Corporation in the 1800s. They were interested in cannabis and psychosis. But the big studies came out about 20 years ago, and they were from Sweden. People at 18 in Sweden, they go for national service and they have quite a thorough medical exam and questionnaire. And they ask people things like, "How many drugs have you taken by the age of 18?" And the most interesting thing for me is that people actually answer them—and answer them truthfully—which would not necessarily happen in many other countries. Because what they did in this study is, having done that, they then took the results of that medical exam and they linked it to people's health service use over the rest of their life. And they found that the amount of cannabis that you have taken before the age of 18 was directly linked to your risk of developing schizophrenia. If you take cannabis one or two times it did not seem to make too much difference. But after that, the more cannabis you took, the more your risk of developing a psychosis.
So following that, there has been quite a lot of research trying to work out what this means. And what people have found is that cannabis seems to be toxic to the brain. It not only increases your chance of developing psychotic symptoms, but it also has direct effects on the structure of the brain. If you are a person with a family history, taking cannabis increases your chance of developing psychotic symptoms, but it also directly damages your brain, it shrinks your brain. So cannabis is actually very important and very linked to the development of psychosis.
What a lot of people then say is, "Well that's fine, because there is no family history of psychosis in my family. I will just take the cannabis and there will not be a problem." What they forget is the level of stigma that there has been in the past. You may well have a family history of psychosis and no one will have told you. People used to make sure that this was really not passed down the generations. That nobody told anybody that Aunt Maud or Uncle Harry or something had a psychotic break at some time. So when you take cannabis, it is like playing Russian roulette. You don't know whether you are the person who has a vulnerability and who is going to develop psychosis.